1. Field of the Invention
The present invention relates to a laser irradiation method, a method for manufacturing a semiconductor device, which uses the laser irradiation method, and a laser irradiating system, and particularly to a technique that can be utilized in processing a thin film such as a semiconductor film.
2. Description of the Related Art
In recent years, the technique for forming a thin film transistor (hereinafter referred to as TFT) over a substrate has made great progress and the application to an active matrix type semiconductor device has been developed. Especially, a TFT with a polycrystalline semiconductor film is superior in field-effect mobility (also called mobility) to a TFT with a conventional amorphous semiconductor film, and thereby a high-speed operation is possible. Therefore, it has been tried to control a pixel by a driver circuit formed on the same substrate as the pixel, which was controlled conventionally by a driver circuit provided outside the substrate.
By the way, a substrate used for a semiconductor device is expected to be a glass substrate rather than a single-crystal silicon substrate in terms of costs. However, a glass substrate is inferior in heat resistance and easy to change in shape due to heat. Therefore, in the case of forming a polysilicon TFT over a glass substrate, laser annealing is used for crystallizing a semiconductor film in order to prevent the glass substrate from changing in shape due to heat.
There are, as characteristics of laser annealing, that processing time can be drastically shortened compared with another annealing that utilizes radiation heating or conduction heating and that a semiconductor substrate or a semiconductor film is heated selectively and locally to hardly damage the substrate thermally.
It is noted that the laser annealing described here includes the technique for recrystallizing a damaged layer or an amorphous layer formed over a semiconductor substrate or in a semiconductor film, and the technique for crystallizing an amorphous semiconductor film formed over a substrate. Moreover, the technique for planarizing or modifying a surface of a semiconductor substrate or a semiconductor film is also included.
According to how to oscillate, laser oscillators used for laser annealing are classified broadly into two types, a pulsed laser oscillator and a continuous wave (CW) laser oscillator. In recent years, it has been known that in crystallization of a semiconductor film, a grain size of a crystal formed in the semiconductor film is larger in the case of using a CW laser oscillator than using a pulsed laser oscillator. When the grain size in the semiconductor film becomes larger, the number of grain boundaries included in a channel region in a TFT formed with the semiconductor film decreases to obtain a higher mobility. As a result, the TFT can be applied to a high-performance device. Accordingly, the CW laser oscillator is beginning to attract attention.
Moreover, in performing laser annealing to semiconductor or a semiconductor film, the following method is known: a laser beam emitted from a laser oscillator is shaped into a linear or elliptical beam spot with an aspect ratio of 10 or more (since an ellipse with an aspect ratio of 10 or more looks like a line, the elliptical is called the linear in the specification) through an optical system and the beam spot is scanned to a surface to be irradiated (an irradiated surface). The method enables irradiating the laser beam effectively to a substrate to increase mass-production ability. Therefore, the method is preferably employed for industrial purposes (Japanese Patent Laid-Open Hei8-195357, for example).
In order to effectively perform laser annealing to a semiconductor film formed over a substrate, the following method is employed: a laser beam emitted from a CW laser oscillator is shaped through an optical system to have a linear beam spot at an irradiated surface, which is irradiated to the semiconductor film. Moreover, a scanning stage that has the substrate set is often moved in the direction of a minor axis of the linear beam spot to perform laser annealing to the semiconductor film. The size of the beam spot formed from the CW laser beam is extremely small, and even though green laser that outputs 10 W, which is almost the highest output among the laser oscillators that output wavelengths absorbed in the semiconductor film, is employed, the beam spot becomes an oblong with a size as small as 500 μm×20 μm. By moving the beam spot back and forth and side to side over a surface to be irradiated, laser annealing is performed to a necessary region of the surface to be irradiated.
In this case, since the heavy scanning stage moves at a high speed (between 100 mm/s and 2000 mm/s approximately), vibration is caused due to the movement of the scanning stage. When the vibration transmits to a vibration isolator where an optical system that forms a beam spot and a system are mounted, a laser irradiation track formed on the substrate, which is not linear any more, is undulating in a reflection of the vibration. When the laser irradiation track is undulated, there are formed some portions where the overlapping ratio is extremely high, and some portions where the laser beam is not irradiated at all between the adjacent laser irradiation tracks formed according to the back-and forth motion of the scanning stage. Since a TFT is formed over the substrate in an orderly arrangement, a TFT formed in the above-mentioned portions is inferior in electrical characteristics, which also causes fluctuation in electrical characteristics. It is a first object of the present invention to suppress the undulation of the irradiation tracks due to such vibration.
In addition, FIG. 1 shows an irradiation track of a beam spot 111 on a semiconductor film. In the irradiation track of the beam spot 111, states of crystals, which can be broadly classified into two types, are formed. In regions A and C, a state similar to crystals formed in the case of performing laser crystallization with pulsed excimer laser is formed. On the other hand, in region B, another state of crystals, in which grain size are larger than those in the case of the crystallization with the pulsed excimer laser, (hereinafter, this state is called a large grain size) is formed.
When the grain size in the semiconductor film becomes large, the number of grain boundaries in a channel region of a TFT formed with the semiconductor film decreases to obtain a higher mobility. On the contrary, the mobility of a TFT in the region where the state similar to the crystals formed in the case of performing laser crystallization with excimer laser is formed, is much inferior to the mobility of a TFT in the region of the large grain size. That is to say, a big difference is caused between electrical characteristics of a TFT formed in the region of the large grain size and a TFT in the region where the state similar to the crystals formed in the case of performing laser crystallization with excimer laser is formed. The difference causes fluctuation in electrical characteristics in the substrate. It is a second object of the present invention to minimize, as much as possible, the region where the state similar to the crystals formed in the case of performing laser crystallization with excimer laser is formed, which is formed in the semiconductor film.